The field of the present invention is polyamide powders for the coating of metals.
The invention is particularly concerned with methods for the production of polylaurolactam compositions for the coating of metals at high temperatures obtained by grinding low molecular weight brittle polylaurolactam with subsequent heating of the powder to temperatures of below its melting point.
The state of the prior art of producing polyamide powders may be ascertained by reference to U.S. Pat Nos. 2,698,966; 2,742,440; 2,975,128; 3,203,822; 3,299,009; 3,476,711; 3,927,141; and 3,966,838; British Pat Nos. 358,138 and 830,757; West German Published Application Nos. 1,570,392; 1,669,821; 1,210,183; 1,267,428; 1,520,551 and 1,495,147; and the Kirk-Othmer "Encyclopedia of Chemical Technology" 2nd Ed., Vol. 16 (1968), under the section "Polyamide (Plastics)", pages 88-105, particularly page 92--polylauryllactam (nylon-12), and polyundecanamide (nylon-11), page 101 Solution Processes, and Powder Processing, pages 101-102, the disclosures of which are incorporated herein.
U.S. Pat No. 2,698,966 discloses physical mixtures of different types of nylon powders produced by dissolving the nylon in organic solvents and precipitating the powdered particles. In Example 14 of U.S. Pat. No. 2,742,440, the solution of epsilon caprolactam in alcohol-water solution and its precipitation as a powder is disclosed. The dispersion of Teflon powder in a solution of epsilon caprolactam and the coprecipitation of the powders is disclosed in U.S. Pat. No. 2,975,128.
The flame spraying and fluidized bed coating of nylon on a metal base is disclosed in U.S. Pat. No. 3,203,822. U.S. Pat. No. 3,299,009 discloses the n-methoxymethylation of nylons, and U.S. Pat. No. 3,410,832 discloses the preparation of polymers and copolymers of lauryllactam.
It is known to use polyamide powder coating compositions for the preparation of varnish-type coatings on metals. The coating operation is conducted in accordance with the fluidized bed coating method, the flame spraying method, or the electrostatic coating method. The polyamide powders are obtained by precipitating the polyamide from solutions, or by grinding the polyamide granules, preferably at low temperatures under an inert gas atmosphere.
It is furthermore known to produce polyamide powders by grinding low molecular polyamides and then bringing the thus-obtained powders to the desired relative viscosity or molecular weight by heating them conventionally to temperatures of below the melting point as disclosed in British Pat. No. 535,138; West German Published Application No. 1,570,392 and U.S. Pat. No. 3,476,711.
Polylaurolactam powders are also prepared in accordance with these conventional methods and are used for coating purposes according to known methods as disclosed in Chem. Ind., November 1968: 783-791, and Modern Plastics, February 1966: 153-156. Since polylaurolactam powders do not always meet the required conditions of high elasticity, satisfactory edge coating, smooth surface, resistance to alkaline aqueous solutions, and in many cases tend especially to smoke during processing, a large number of improvements have become known, such as plasticizer containing polylaurolactam powders, as disclosed in West German Published Application 1,669,821, those of a mixture of homopolylaurolactam and laurolactam-containing copolyamides, as disclosed in U.S. Patent Application Ser. No. 283,966, those containing polyamides with N-alkoxymethyl groups in addition to acid reacting catalysts, as disclosed in U.S. Pat. No. 3,966,838, or mixtures of polyamides having 8-11 aliphatically bound carbon atoms per carbonamide group, aminoplasts carrying alkoxyalkyl groups, and acid reacting catalysts, as disclosed in U.S. Pat. No. 3,927,141. These powders exhibit good properties in individual cases, but do not as yet fully satisfy all of the required conditions.